An "initcall" is the initialization function for a module which is statically linked into the Linux kernel. Running of initcalls can consume a large amount of time during bootup. However, some modules do not need to be initialized so early, in order for the embedded device to become usable for its primary purpose. These initcalls can be run later, after first boot (and possibly first use) has completed.
Digital Camera Example
For example, many digital cameras have USB buses, which need to be initialized in order for the camera to be used as a mass storage device (to download pictures to a desktop computer). However, the the USB buses do not need to be initialized for the primary use of the camera, which is to take pictures. In order to be able to take a picture as soon as possible after booting, initialization of the USB system in the kernel can be deferred until after the primary boot sequence for the camera.
Using a short patch (available for kernel version 2.6.27) it is possible to avoid running certain initcalls at bootup time. The way this patch works is that the developer selects some initcalls that they want to defer and modifies the declaration of those initcalls.
When the system is booted, those initcalls are not called in their normal sequence. When the system is done with primary boot, an application from user space triggers the calling of the deferred initcalls, using a flag in the /proc filesystem.
When the flag is set, the deferred initcalls are run, and the kernel memory for the "init" segment is finally freed.
How to use
To use deferred initcalls, first you need to identify the modules that are not required for the primary functionality of the product, and which are taking too long to initialize. (See Initcall Debug for this.)
Using the example above of the digital camera and USB, you could identify uhci_hcd_usb and ehci_hcd_init as two initcalls that could be deferred.
Change the module init routine declaration for the initcalls you wish to defer. This is done in the Linux kernel source code. For example, change:
Modules marked like this are not initialized during kernel boot
After main init, do:
echo 1 >/proc/deferred_initcalls
This will cause the kernel to run all deferred initcalls. Also the .init section memory is freed by kernel.
deferred USB initcall example
As a test, on an X86 desktop system, I deferred the initialization of the USB subsystem on a 2.6.27 kernel, by using deferred_module_init on the functions: ehci_hcd_init and uhci_hcd_init
This resulted in a total times savings of 530 milliseconds, during the kernel boot phase. (Of course, this time was used subsequently when the deferred initcalls were triggered later on.)
Specifially, I changed: